This page provides an independent statistical analysis and educational interpretation of publicly reported results. ClinicalTrials.gov provides the official trial registry record. Numerical trial results on this page are restricted to the results posted in the ClinicalTrials.gov record; calculations added by Clinical Biostats are labelled as such.
1. Trial at a Glance
This page covers the registry results for Immunogenicity & Reactogenicity of Boostrix 10 Years After Previous Booster Vaccination. (NCT00610168), a non-randomized phase 4 study. The primary outcome is Number of Subjects With Anti-diphtheria (Anti-DT) and Anti-tetanus Toxoids (Anti-TT) Antibody Concentrations Above the Cut-offs.
| Feature | Registry value |
|---|---|
| ClinicalTrials.gov identifier | NCT00610168 |
| Phase | Phase 4 |
| Condition | Acellular Pertussis, Tetanus, Diphtheria |
| Allocation | Non-Randomized |
| Design model | Parallel |
| Masking | None (open label) |
| Enrollment | 82 |
| Arms in the primary outcome | n/a |
| Lead sponsor | GlaxoSmithKline |
| Results posted | Yes |
2. Clinical Question
The study asks whether adults who had received a prior booster in an earlier clinical study have measurable antibody concentrations against diphtheria and tetanus at the Month 0 assessment, and describes antibody measures and safety outcomes in the context of a further Boostrix booster. The primary endpoint counts subjects whose anti-diphtheria or anti-tetanus antibody concentrations meet stated thresholds. The record also lists pertussis antibody measures and solicited and unsolicited adverse events among the secondary outcomes.
Population
Young adults eligible because they received a booster in the previous clinical study. The primary immunogenicity analysis population is described as evaluable subjects who received Boostrix and had immunogenicity data available.
Intervention
One additional dose of Boostrix. The first group description says the vaccine was administered by intramuscular injection into the deltoid region of the non-dominant arm; the description of the second group is incomplete in the registry record.
Comparator
The study is non-randomized and has two groups defined by the vaccine history in the earlier study. Both groups received Boostrix in this study, so they are not a like-for-like control comparison.
Primary question
How many subjects had anti-diphtheria and anti-tetanus antibody concentrations at or above the stated cut-offs at Month 0? The registry record gives the endpoint definition but does not report the resulting counts.
This is an immunogenicity and reactogenicity question rather than a randomized comparison of booster versus no booster. The record’s stated purpose includes assessing efficacy and safety, but the listed primary measure is antibody concentration status, not a clinical disease outcome. Consequently, the endpoint definition alone cannot establish how antibody levels changed after vaccination or whether a clinical outcome differed between groups. The record does not report the primary endpoint results, so the question can be described from its protocol entry but not answered from the posted results.
3. Trial Design
The registry identifies this as a completed Phase 4 study with non-randomized allocation and no masking. It used a parallel-group model and enrolled 82 participants. The record gives a start date of 2008-01-01 and a primary completion date of 2008-04-30. Its stated context is a repeat dTpa booster in adults 10 years after an earlier dTpa booster, with eligibility restricted to people who received a booster in the previous clinical study.
The groups reflect different vaccine histories in that earlier study. The BOOSTRIX I GROUP had received Boostrix in the primary study; the BOOSTRIX II GROUP had received a combined adult diphtheria and tetanus vaccine and a separate acellular pertussis vaccine. Both group descriptions indicate a Boostrix booster in the current study. The description of the second group’s current-study administration is cut off in the registry record, so further details should not be inferred.
Because allocation was not randomized and both groups received a booster in this study, their contrast is not equivalent to a randomized treatment-versus-control comparison. Differences between their outcomes could reflect their different earlier vaccine histories or other group differences, as well as variation. The record does not report a matching or adjustment strategy that would address such differences.
The registry lists 10 outcome measures, including the primary antibody-threshold count and secondary antibody, booster-response, and safety outcomes. It lists no analyses. The statistical plan, any sample-size justification used to design the trial, and any interim analyses are not reported in the registry record. Planning illustrations based on an observed effect are not evidence of the original design rationale, nor are they observed power.
4. Participant Flow and Accounting
The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.
Before assignment: During the screening the following steps occurred: check for inclusion/exclusion criteria, contraindications/precautions, medical history of the subjects and signing informed consent forms.
Overall Study
| Stage | Boostrix I Group | Boostrix II Group |
|---|---|---|
| STARTED | 75 | 7 |
| COMPLETED | 73 | 7 |
| NOT COMPLETED | 2 | 0 |
Reasons for not completing this period:
| Reason | Boostrix I Group | Boostrix II Group |
|---|---|---|
| Withdrawal by Subject | 2 | 0 |
Analysis population for the primary outcome, as defined in the registry: The analysis was performed on the According-to-Protocol (ATP) cohort for analysis of immunogenicity, which included all evaluable subjects who had received the booster dose of Boostrix™ vaccine and for whom immunogenicity data were available.
5. Baseline Characteristics
The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.
| Characteristic | Unit | Boostrix I Group | Boostrix II Group | Total |
|---|---|---|---|---|
| Participants analysed | 75 | 7 | 82 | |
| Age, Continuous | Years | 21.1 (SD 0.31) | 21.1 (SD 0.38) | 21.1 (SD 0.33) |
| Sex: Female, Male: Female | Participants | 66 | 6 | 72 |
| Sex: Female, Male: Male | Participants | 9 | 1 | 10 |
6. Primary Endpoint
The registry lists the primary outcome as Number of Subjects With Anti-diphtheria (Anti-DT) and Anti-tetanus Toxoids (Anti-TT) Antibody Concentrations Above the Cut-offs, measured over At Month 0.
| Element | Registry entry |
|---|---|
| Result type | Number |
| Measure of spread | not applicable |
| Unit | Subjects |
| Endpoint classification | Binary Endpoint |
The primary endpoint is the number of subjects with anti-diphtheria and anti-tetanus antibody concentrations above specified cut-offs at Month 0. The record names cut-offs equal to or above 0.1 international units per milliliter (IU/mL) and equal to or above 1 IU/mL. The analysis population is the According-to-Protocol cohort for immunogenicity: evaluable subjects who received Boostrix and had immunogenicity data available.
In general, an antibody threshold turns a measured concentration into a binary classification: each eligible subject either meets a specified threshold or does not. A count can describe how many subjects meet the criterion, while a proportion can make the result easier to interpret relative to the number assessed. The registry record reports the endpoint definition, but not the counts, denominators, or uncertainty estimates for these classifications.
Antibody thresholds are used as laboratory measures of immune response; they are not, by themselves, direct observations of disease prevention. The record does not state that the threshold values are validated clinical correlates for the outcomes of interest, and it does not report a clinical disease endpoint. Nor does the Month 0 label alone establish when the measurement occurred in relation to the booster; that timing detail is not reported in the registry record. The endpoint therefore identifies what was to be counted, but cannot establish a post-booster change or a clinical effect from the available result information.
7. Why No Re-analysis Is Shown
8. Statistical Interpretation
The main interpretive constraint is that the registry record provides an endpoint definition without the primary outcome values. It states that the endpoint counts subjects meeting anti-diphtheria and anti-tetanus antibody thresholds at Month 0, but gives no observed counts or denominators for those measures. No confidence intervals or test results are reported. The posted information therefore does not quantify the primary immunogenicity result or its precision.
Other outcome names describe antibody concentrations, seropositivity, booster response, and adverse events. However, an outcome being listed is not the same as its result being reported. The record lists no analyses, and does not provide numerical results for the secondary immunogenicity outcomes. The safety information includes a pooled group with 82 subjects at risk for serious adverse events, 1 subject affected, and 78 subjects affected for other adverse events. These figures describe the recorded group counts; the record does not give the detail needed to compare adverse-event risks across the two prior-vaccine-history groups.
The study’s non-randomized allocation matters when reading any group contrast. In general, random allocation helps balance measured and unmeasured participant characteristics between groups. Without randomization, observed differences can be associated with prior vaccine history or other group differences, and cannot automatically be attributed to the current booster. Here, both groups received Boostrix, so the design does not supply a concurrent no-booster control.
The registry does not report an inferential analysis, a statistical analysis plan, or uncertainty estimates. In general, a point estimate gives the observed value, while a confidence interval conveys the range of values compatible with the data under the method’s assumptions. Without either the endpoint results or their uncertainty, the precision of the primary estimate cannot be assessed. The sample-size assumptions describe planning illustrations based on an observed effect, not the trial’s design justification and not observed power. Overall, the record supports a description of the intended measures and limited pooled safety counts, but not a quantitative conclusion about the primary endpoint, group differences, or clinical outcomes.
9. Statistical Methods Explained
The registry record names a binary primary outcome: the number of subjects meeting antibody cut-offs. It does not report an analysis method or results. The explanations below are general statistical context, not a description of methods confirmed for this trial.
What does a threshold-based outcome represent?
For a threshold-based endpoint, a laboratory measurement is classified according to whether it reaches a specified cut-off. The count of subjects meeting that cut-off depends on both the observed measurements and the population with usable measurements. A proportion adds the denominator, which is needed to understand the count in context. A count without its denominator can be difficult to compare across populations or outcomes, and the registry does not report the primary counts or denominators.
The primary definition includes anti-diphtheria and anti-tetanus measures and two cut-offs. These are distinct endpoint components: a result for one antibody or cut-off would not automatically establish the result for another. The record does not explain how these components were combined, whether any one was designated as the main inferential test, or how comparisons across them were handled.
How should uncertainty be understood?
In general, an estimate from a study is subject to sampling variation. A confidence interval is a way to communicate uncertainty around an estimate, given a specified statistical method and its assumptions. A narrower interval indicates greater precision under that method; it does not remove possible bias from study design, measurement, or missing data. No confidence intervals are reported for the primary endpoint in the registry record, so its statistical precision cannot be evaluated.
A hypothesis test, when used, evaluates how compatible the data are with a stated null hypothesis under a model. A p-value is not the probability that a treatment works, nor does it measure the size or importance of an effect. The registry record does not report p-values, hypothesis tests, or the hypotheses that would have been tested. No inferential conclusion should be inferred from an endpoint title or from the fact that the study is marked completed.
What can group comparisons establish?
In general, non-randomized groups can differ in characteristics other than the exposure or history used to define them. Adjustment for measured factors can sometimes address some differences, but it cannot guarantee that all confounding has been removed. The registry describes groups by the vaccine regimens received in the prior study and indicates that both received Boostrix in this study. It does not report a statistical adjustment strategy. Any difference between group outcomes would therefore need cautious interpretation and would not, on its own, establish that the current booster caused the difference.
The parallel-group structure means the groups are followed as distinct sets of participants. It does not make their comparison randomized. The registry also states that masking was absent. It does not report whether any particular outcome assessment could have been influenced by knowledge of group or treatment, so the potential relevance of lack of masking cannot be quantified from this record.
What do planning calculations mean?
In general, sample-size planning uses assumptions about the effect to detect, variability or event frequency, allocation, and the chosen error rate. A planning illustration can show how the required participant count changes under different assumed effect sizes. The record describes illustrations based on a fraction of the observed effect, with a two-sided significance level of 0.05, equal allocation, and a normal approximation. It also says that the event-based calculations use total events, while other calculations use participants per arm.
Those assumptions describe planning illustrations, not a confirmed design plan for this trial. The registry does not report its own sample-size justification. A calculation based on an observed effect is not observed power, and it cannot replace the missing endpoint result or its uncertainty. The registry lists no analyses, so there is no reported analysis output to assess against those planning assumptions.
Why can over-dispersion matter?
In general, over-dispersion occurs when count data vary more than a chosen statistical model expects. If it is ignored, estimated uncertainty can be too small and statistical tests can be poorly calibrated. Whether this issue applies depends on the outcome and model. The primary endpoint is defined as a count of subjects meeting thresholds, but the registry does not report a model, dispersion estimate, or method for handling over-dispersion. No conclusion about its relevance to the trial’s analysis can be drawn.
10. Multiplicity, Missing Data and Analysis Populations
The registry lists multiple immunogenicity outcomes: anti-diphtheria and anti-tetanus thresholds, pertussis antibody concentrations and seropositivity, and booster-response counts. It also lists several safety outcomes. Considering multiple outcomes or thresholds can increase the chance of at least one apparently notable finding when many statistical tests are performed. In general, a prespecified multiplicity strategy can control how results are interpreted across a set of tests. The strategy for this study is not reported in the registry record.
The record does not identify a hierarchy among the primary endpoint’s antibody and threshold components, state whether tests were planned for each component, or explain any adjustment for multiple comparisons. It also lists no analyses. Therefore, the number and nature of statistical tests actually performed, and any adjustment applied, are not reported in the registry record. The listed outcomes should not be treated as a collection of statistically confirmed findings.
The immunogenicity population is described as evaluable subjects who received the booster and had immunogenicity data available. This indicates that availability of data forms part of the stated analysis-population definition, but the record does not give the number included in that analysis or the reasons some participants might not have been evaluable. A missing-data method, including any imputation or sensitivity analysis, is not reported in the registry record.
In general, excluding participants without measurements can affect interpretation if missingness is related to participant characteristics or outcomes. The extent and direction of any such effect cannot be determined without details about missingness and analysis. The record also does not specify a safety analysis population or give outcome-specific denominators beyond the pooled adverse-event figures. These omissions limit assessment of both multiplicity and the representativeness of the reported analyses.
11. Safety Results
The registry reports serious adverse events as affected participants divided by participants at risk.
| Arm | Participants with serious adverse events | Participants at risk | Percentage |
|---|---|---|---|
| Boostrix Pooled Group | 1 | 82 | 1.2% |
Other (non-serious) adverse events
Participants with at least one non-serious event at or above the registry's 5% reporting threshold.
| Arm | Participants affected of at risk |
|---|---|
| Boostrix Pooled Group | 78 of 82 |
Most frequent serious adverse event terms
| Event term | Organ system | Boostrix Pooled Group |
|---|---|---|
| Hyperventilation | Respiratory, thoracic and mediastinal disorders | 1 of 82 |
Most frequent non-serious adverse event terms
| Event term | Organ system | Boostrix Pooled Group |
|---|---|---|
| Pain | General disorders | 76 of 82 |
| Redness | General disorders | 48 of 82 |
| Swelling | General disorders | 46 of 82 |
| Fatigue | General disorders | 44 of 82 |
| Headache | General disorders | 27 of 82 |
| Gastrointestinal symptoms | General disorders | 14 of 82 |
| Fever (Axillary) | General disorders | 7 of 82 |
| Influenza | Infections and infestations | 6 of 82 |
Serious events with the highest arm-level rate: hyperventilation (1.2%).
The registry lists a pooled Boostrix group with 82 subjects at risk for serious adverse events and 1 subject affected. It identifies hyperventilation at 1.2% among the top serious adverse-event terms. The record also lists 82 subjects at risk and 78 affected for other adverse events. These are descriptive counts; the record does not provide enough detail to interpret differences between the two groups defined by prior vaccine history.
In general, an adverse-event count describes events observed and recorded within a stated population and observation period. It does not by itself show that the intervention caused an event. Attribution requires clinical and study-context information, and comparisons require compatible denominators, follow-up, and methods. The registry does not report an analysis that estimates comparative safety between the groups, and it does not provide a detailed account of event timing or severity beyond the listed outcome categories.
The death field is not populated, so deaths are not reported in the registry record; that should not be interpreted as evidence that none occurred. Likewise, a pooled count cannot establish whether events were distributed evenly between groups. The adverse-event figures should be read as limited descriptive information, not as a causal or comparative safety conclusion.
12. What the Result Does, and Does Not, Mean
The record describes a completed study of a further Boostrix booster among adults eligible through participation in an earlier clinical study. It specifies a primary immunogenicity endpoint based on anti-diphtheria and anti-tetanus antibody thresholds and lists secondary antibody and safety measures. The pooled safety information includes 1 subject with a serious adverse event among 82 at risk, as well as 78 subjects affected for other adverse events among 82 at risk.
Those details do not establish the primary immunogenicity result. The registry does not report the number of subjects meeting the antibody thresholds, the corresponding denominators, or uncertainty estimates. It also does not report numerical results for the other immunogenicity outcomes. The endpoint definition describes what was to be measured, not the observed result.
The study design also limits causal interpretation. Allocation was non-randomized, the groups differed by vaccine history in the earlier study, and both groups received Boostrix in the current study. A contrast between groups would not be equivalent to a randomized comparison of booster versus no booster. The registry does not report an adjustment strategy or analysis that could address potential group differences.
In general, antibody measures are laboratory outcomes, whereas clinical protection is a separate question that requires relevant clinical evidence. The record does not report a clinical disease outcome or establish how the specified antibody thresholds relate to such outcomes. It therefore cannot support a conclusion about disease prevention. It also does not provide enough detail to characterize the completeness of safety follow-up or to determine whether the pooled counts capture every relevant event. The clearest interpretation is limited: the record defines the intended measurements and reports selected pooled safety counts, but does not provide the outcome results or analyses needed for broader efficacy, comparative, or causal conclusions.
13. Why This Trial Matters Statistically
This trial illustrates an important distinction between a study’s stated question and the information available to evaluate its answer. A registry can describe an endpoint precisely, including its population, measurement, threshold, and assessment time, while still lacking the numerical results needed to estimate the observed outcome. Here, the primary endpoint is defined, but the number meeting its antibody cut-offs and its precision are not reported.
The study also shows why design labels matter. A parallel-group structure does not imply random allocation, and two groups do not automatically form a valid control comparison. The group histories differ from the earlier study, while both groups received Boostrix in this study. This context must be considered before interpreting any apparent difference between groups.
Another statistical lesson is that a list of many endpoints does not tell the reader how they were analyzed. Multiple antibodies, thresholds, response definitions, and safety outcomes raise questions about which comparisons were central and how multiplicity was handled. The registry does not report that plan or any analyses. This makes it impossible to distinguish prespecified inferential results from exploratory assessments.
Finally, planning calculations and observed results serve different purposes. Planning illustrations can explore participant requirements under stated assumptions, but they do not reveal whether a study’s actual outcome was precise, statistically persuasive, or clinically meaningful. This record’s planning assumptions cannot fill the gap left by missing endpoint results. The case is therefore useful for reading registry entries critically: study status, endpoint definitions, and planning assumptions are not substitutes for complete result reporting and a transparent analysis plan.
14. Limitations
- The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
- Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.
The registry record does not report the primary endpoint counts, denominators, confidence intervals, or test results. It lists 10 outcome measures but no analyses. This leaves the primary immunogenicity outcome and the secondary outcomes without numerical result detail in the record.
The study is non-randomized, and the two groups are defined by different vaccine histories in the earlier study. Both received Boostrix in the current study, so there is no concurrent untreated group described. Any group contrast would be vulnerable to differences associated with prior history or other characteristics; an adjustment strategy is not reported.
The immunogenicity analysis population is restricted to evaluable participants with available data who received the booster, but the record does not state the number included in that population or describe missingness. The safety figures are pooled, so group-specific counts and comparative estimates are not available. The record also does not report detailed follow-up information for the adverse events or a populated death count.
The second group’s intervention description is incomplete in the registry record. The statistical analysis plan, sample-size justification, interim analyses, multiplicity handling, and method for missing data are not reported in the registry record. These limitations prevent assessment of whether the analyses followed prespecified methods, how robust the findings would be to missing data, and how precisely outcomes were estimated. The record’s stated purpose mentions efficacy and safety, but the listed primary outcome is an antibody measure; clinical disease results are not reported.
15. Related Tutorials
16. Related Statistical Calculators
17. Sources
- ClinicalTrials.gov: Immunogenicity & Reactogenicity of Boostrix 10 Years After Previous Booster Vaccination. — NCT00610168.
- Registry record last updated 2018-06-06; status last verified 2017-02.
- Results first posted 2017-03-30.
- Data retrieved from ClinicalTrials.gov on 2026-11-09.
- Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.
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18. Record Summary
The registry identifies a completed Phase 4, non-randomized, unmasked, parallel-group study of Boostrix as a repeat booster in adults 10 years after an earlier booster. It enrolled 82 participants. The groups are distinguished by the vaccine regimens received in the earlier study, and both received Boostrix in the current study.
The primary endpoint counts subjects meeting anti-diphtheria and anti-tetanus antibody cut-offs at Month 0. The record defines the immunogenicity analysis population but does not report primary endpoint counts, denominators, or uncertainty estimates. It lists 10 outcome measures and no analyses.
For pooled safety information, 1 subject among 82 at risk had a serious adverse event, and 78 subjects among 82 at risk had other adverse events. Hyperventilation is listed at 1.2% among the top serious adverse-event terms. The record does not report a statistical analysis plan, multiplicity or missing-data methods, or the trial’s sample-size justification.